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Yeast, like all living organisms, have a narrow range of temperature at which their growth is optimal. Temperatures below this range tend to slow down or even stop metabolic processes, while higher temperatures can cause cell death.

For many fermentation processes, different temperatures significantly impact product quality.

Ale beer is fermented at a relatively warm temperature (18-24°C) while lager fermentations require lower temperatures (5-15°C). Similarly, red wine is generally fermented at a warmer temperature than the white wine. Fermentation at different temperatures is only possible due to the availability of a variety of yeast species/strains that are adapted to specific temperature conditions.

The yeast Saccharomyces cerevisiae is the most commonly used species for baking, wine, cider and ale fermentations, but is sensitive to low temperatures. When there is a requirement for very low temperature fermentation this species is commonly found in hybrid form with a cold-tolerant species. This is the case for lager yeast.

Through hybridization, lager yeast was able to combine the good fermentative behavior of S. cerevisiae and the cold tolerance of the wild yeast S. eubayanus.

In 2015, VTT generated the first new lager brewing yeast strains in 500 years, proving that it is possible to generate hybrids in a targeted way for specific industrial processes. We recently applied the knowledge obtained in that work to create new yeast strains for the production of cider and wine.

The cold-tolerant Saccharomyces eubayanus

Cold fermentations have some advantages for cider and wine making. At such temperatures, the risk of contamination is low, reducing the need to use sulphite as a preservative. In addition, the aroma profile of white wines is generally improved by using lower fermentation temperatures. White wine and cider are already fermented in relatively cold conditions; therefore, the use of a cold tolerant yeast can potentially improve the process.

The wild yeast S. eubayanus has proven to be one of the most cold-tolerant yeasts known and is partly responsible for the success of the lager beer. In addition, previous results suggest that it contributes to a more diverse aromatic profile and it has good ability for fructose transport. Fructose represents 2/3 of the fermentable sugars in apple juice and half of the sugars in grape must. Due to the total sugar concentration being considerably higher in grape must, incomplete fructose utilization often becomes a limiting step. Therefore, the cold tolerant S. eubayanus has several phenotypic traits relevant for cider and wine making. However, the use of this species is so far limited to lager beer (in hybrid form) and to the spontaneous fermentation of Mudai (a traditional Patagonian beverage prepared with the seeds of the monkey puzzle tree).

New hybrid strains tolerate a wide range of temperatures

We successfully crossed this cold tolerant yeast with a S. cerevisiae wine strain, and tested the fermentative potential in cider and wine making. One of the most important observations was that the new hybrid yeasts tolerated a wide range of temperatures (10-37°C). The lower range allows the yeast to efficiently ferment at low temperatures, while the higher temperatures allow the yeast to resist the stressful conditions associated with the large-scale production in active dry yeast form.

The wider range of temperatures also means that it can serve multiple purposes. Temperature greatly impacts the aromatic properties of cider and wine, and therefore, the aromatic properties can be modulated by simply altering fermentation temperature.

Significantly more pleasant ciders

To test the new hybrid strains we first fermented apple juice at 10 and 20°C. The hybrid yeast outperformed the parental wine yeast. Particularly at 10°C, the wine strain was seriously inhibited by the low temperature. In addition, the hybrids produced more esters, giving a fruity/floral flavour to the ciders. Importantly, unpleasant volatile compounds produced by the wild parent were also eliminated through hybridization.

The sensorial properties of the ciders were evaluated by a consumer panel at the Functional Food Forum of the University of Turku, confirming that the ciders produced by the hybrids were perceived to be significantly more pleasant than those produced by the cold tolerant parent alone.

Wines with more complex and exotic aromatic profiles

To test the potential of the hybrid yeast for wine making, we fermented Sauvignon Blanc and Maccabeu grape musts at laboratory scale. The fermentation performance was comparable to commercial wine yeast strains but with a stronger, more complex and exotic aromatic profile, as described by a panel of expert tasters.

Furthermore, the best performing hybrid yeast was tested for its capacity to maintain high viability after drying and rehydration. The results were very positive with viabilities comparable or even higher than yeast strains commercialized in dried form.

Improved baking yeasts next

We have demonstrated that hybridization is a very powerful technique to improve yeast performance and phenotypic diversity for applications where targeted genetic modification is not yet accepted. After successfully applying this technique to beer, wine and cider production, we are now assessing hybrids for their potential use by the baking industry. This is being done with the support of a Key Projects grant from the Academy of Finland.

Cold tolerance is also a critical issue in baking. The advent of frozen dough use in recent years represents a challenge for baking yeast. Yeast lose viability when stored frozen, and the poor heat conductivity of dough means that yeast can be exposed to quite low temperatures during the proofing stage of baking. A yeast strain able to perform well at a wider range of temperatures might aid in making loafs of bread evenly fermented after freezing.

The author Frederico Magalhães is a third-year PhD student at VTT’s Bioprocess engineering team. His main research interests include studying of yeasts’ natural diversity to develop new hybrid yeasts with relevant phenotypes for the beverage and bread industry. frederico.magalhaes@vtt.fi